Chemistry of rancidity & Olive oil

Food chemistry is one of my favorite topics to discuss, especially when combined with actual food :-)! That’s why this month is food chemistry month. Today we focus on the science of fats, zooming in on rancidity of fats and olive oil.

Food chemistry studies molecules and their reactions, as I posted before. One of the main group of molecules in food are of course fats. Fats are important from a nutritional point of view for storage of energy, but also play a very important role in many other foods. They delay a bread from going stale, give a nice mouthfeel to chocolate or give your tempura batter a great crust.

Today we’re going to zoom in on the chemistry of fats and focus on one specific phenomena, the chemistry of rancidity of fats! To understand why your butter or olive oil can turn rancid, we have to start with some basic chemistry of fats.

Chemical structure of fats/triglycerides

So far I’ve only mentioned fats but in this post I’ll be talking about both fats and oils. There isn’t a big difference between the two from a chemical point of view. The main difference is that oils are liquid are room temperature (olive oil, sunflower oil, rapeseed oil), whereas fats are solid (butter, lard). Whenever I talk about fats here, I also talk about oil (unless I explicitly say otherwise 🙂 ).

The chemical structure of fats (and thus oils) is called a triglyceride. A triglyceride is build up of a molecule called glycerol to which 3 fatty acids are attached (see how appropriate the name triglyceride is?!).

Triglyceride with glycerol and fatty acid
This image shows a triglyceride, with the glycerol molecule and one of the three fatty acids. The A stands for an unknown group, this is always a chain of carbon atoms, but the length can vary. Alls the nodes where several lines come together, represent a carbon atom, these are generally left out from this types of structure to keep an overview.

Chemical structure of fatty acids

There are a lot of different triglycerides, the difference sits in the fatty acids. There are a lot of different fatty acids which each have a different carbon chain in the spot of the A in the molecule above, see below for some examples.

Fatty acids that have double bonds between carbon atoms are called unsaturated fatty acids. The chains that only have single bonds are called saturated fatty acids.

The chains of a fat influence whether this fat is solid at room temperature or liquid, so whether it’s an oil or fat. The longer a chain is and the more bended (for that it requires a double bond) the harder it is for them to pack in a compact way. If it’s harder to order them properly, it will not become solid that easily, so will be a liquid at a higher temperature.

Fatty acids

Cause of rancidity

You might have noticed, when your butter or olive oil gets old, the smell will worsen. This is caused by the fats getting rancid. Two different things happen when your fats or oils get rancid.

First of all, a so called hydrolysis reaction takes place. The fatty acids get separated from glycerol. These fatty acids often don’t smell nice and this is what gives the butter an off odour and flavour.

Another reason for rancidity has to do with the double bonds in unsaturated fatty acids. These can react with oxygen or light in a so called oxidation which can also produce flavours and odours with off smells.

Preventing rancidity

The easiest way to slow down hydrolysis of fats, thus the fatty acids splitting off from the glycerol molecule, is to cool your fats. So place your butter in the fridge and it will take a lot longer to smell bad.

Oxidation can be slowed down/stopped by preventing oxygen or light from coming in contact with your fat. Since olive oil contains quite some unsaturated fatty acids it’s important to keep it in a closed container. The colour of your bottle helps as well here! The reason olive oil is often packed in a green bottle is to prevent excessive light from coming in.

Further reading

Want to know more details? Here are some interesting webpages: a book discussing fats, on fats in general or specifically about the mechanisms.

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